Tuneable cavity resonator

ABSTRACT

The invention concerns a tunable cavity resonator that comprises a resonator body ( 2, 3, 4 ) defining a cavity ( 5 ), a tuning plate ( 28 ) whose position with respect to the resonator body ( 2, 3, 4 ) is modifiable and which influences the resonance frequency (ω R ) of the cavity resonator, and an adjustment device ( 22, 26 ) for mechanically changing the position of the tuning plate ( 28 ), which is characterized in that a conversion ratio mechanism ( 18, 20 ) couples the adjustment device ( 22, 26 ) to the tuning plate ( 28 ) in terms of movement and converts a linear excursion (Δx 1 ) generated by the adjustment device ( 22, 26 ), at a predefined ratio (U), into a reduced linear excursion (Δx 2 ) that acts on the tuning plate ( 28 ), the conversion ratio mechanism ( 18, 20 ) comprising a first spring element ( 20 ) whose end toward the adjustment device is deflectable with the linear excursion (Δx 1 ) generated by the adjustment device ( 22, 26 ), and a second spring element ( 18 ) which impinges with an opposing force on the end of the first spring element ( 20 ) remote from the adjustment device.

FIELD OF THE INVENTION

The invention concerns a tunable cavity resonator as defined in thepreamble of claim 1. The invention further concerns a tunable microwaveoscillator that uses a cavity resonator of such a kind.

BACKGROUND OF THE INVENTION

Tunable cavity resonators are used, inter alia, in microwave oscillatorsthat are utilized to generate carrier signals in microwavecommunication. Such oscillators substantially comprise a microwaveamplifier that is operated in feedback, and a high-quality cavityresonator that is located in the feedback path of the oscillator andfilters out phase noise generated in the amplifier. A microwaveoscillator of this kind furthermore uses a mechanical or electricalphase shifter to adjust the phase condition in the feedback path, and ahigh-frequency coupler to couple out the useful signal (carrier signal).

Adjustment of the oscillator frequency is accomplished in two stages:For coarse adjustment, first the resonance frequency of the tunablecavity resonator is modified in suitable fashion. This is done by meansof the adjustment device with which the position of the tuning platewith respect to the resonator body is displaced. For fine adjustment ofthe oscillator frequency, the oscillator frequency is then shifted incontrolled fashion within the resonance width of the tuned cavityresonator, using the phase shifter to displace the phase in the feedbackpath of the oscillator.

One difficulty with this type of two-stage toning of an oscillatorresults from the fact that the maximum frequency excursion achievable byphase adjustment is relatively small, for example only approximately 100kHz for resonator qualities above 10⁴ (i.e. Q>10⁴). Complete tunabilityof the microwave oscillator can only be achieved, however, if theminimum frequency change achievable in the context of resonancefrequency tuning (i.e. cavity resonator tuning) is less than theaforementioned maximum frequency excursion when varying the phase in thefeedback path of the oscillator. To meet this criterion, cavityresonators with an extremely high tuning accuracy are required.

It must be considered in this context that as the quality Q of a cavityresonator increases, the requirements in terms of the adjustmentaccuracy of the tuning mechanism in order to achieve a defined tuningaccuracy also increase.

In practice, therefore, difficulties often occur in terms of thephysical design of the tuning mechanism; and it has been found that thedesired high adjustment accuracies, in combination with the necessaryvibration resistance and good tuning reproducibility, are not alwaysachieved.

The publication entitled “Temperature compensated high-Q dielectricresonators for long term stable low phase noise oscillators,”Proceedings of the 1997 Frequency Control Symposium, I. S. Ghosh et al.,pp. 1024-1029, describes a tunable cavity resonator as defined in thepreamble of claim 1. This cavity resonator, with a quality Q≈10⁵, meetsthe tuning accuracy requirements necessary for continuous tunability ofa microwave oscillator.

DE 1 687 62 discloses an apparatus for adjusting the spacing between astationary and a movable partition element of a cavity resonator, alever that is in engagement with the movable partition via a bearingelement being arranged rotatably on the stationary partition element.The lever is displaced via a conically tapering segment. The wall of thecavity resonator is thereby moved in order to retune the frequency ofthe resonator. The linear excursion through which the lever travels atits free end is converted at the wall of the resonator into a reducedlinear excursion.

SUMMARY OF THE INVENTION

It is the object of the invention to create a cavity resonator thatpossesses high adjustment accuracy in terms of its resonance frequency.The intention is, in particular, to make available a cavity resonatorthat exhibits high quality and nevertheless makes possible completetunability of a microwave oscillator when used therein. A furtherpurpose of the invention is to create a completely tunable microwaveoscillator having a high-quality cavity resonator.

The features of claims 1 and 12 are provided in order to achieve theobject. The result of the conversion ratio mechanism provided accordingto the present invention is that upon an actuation of the adjustmentdevice, it is not the linear excursion generated by the adjustmentdevice, but rather a linear excursion reduced with respect thereto, thatadjusts the tuning plate. The consequence of this is that the minimumexcursion change attainable with the adjustment device is transformedinto an even smaller minimum excursion change acting on the tuningplate. As a result, the adjustment accuracy of the tuning plate isincreased, compared to the adjustment accuracy of the adjustment device,by an amount equivalent to the predefined ratio of the conversion ratiomechanism. The predefined ratio (i.e. the transmission ratio) isdetermined by the spring constants of the two spring elements. The useof two spring elements pressing against one another has the advantagethat the conversion ratio mechanism operates continuously and in amanner largely free of backlash.

In this instance, a particularly preferred variant embodiment ischaracterized in that the first spring element is formed from at leastone cup spring, and the second spring element is implemented by a platespring that is immobilized at the periphery and impinged upon centrallyby the cup spring. A spring mechanism of this kind can be designed withsufficient stiffness to be insensitive to external shock or vibrations.In addition, suitable cup and plate springs can easily be manufacturedwith the requisite high spring constants.

The adjustment device preferably comprises an, in particular, manuallyactuable mechanical actuating element and a first electromechanicalactuating element, in particular a first piezoelement, downstream fromthe mechanical actuating element. The first electromechanical actuatingelement makes possible electrical activation of the adjustment device,which is advantageous in particular when the adjustment device isoperated in a closed-loop mode for adjustment of the resonance frequencyω_(R). The electromechanical actuating element can also be used, forexample, to compensate for temperature-related drift, and can moreover,within a limited excursion range, eliminate the need for an actuation ofthe mechanical actuating element.

The tuning plate is preferably made of a dielectric material, inparticular sapphire. A tuning plate of this kind has very low dielectriclosses especially at low temperatures, so that the quality achievablefor the cavity resonator (defined as the product of the resonancefrequency OR times the quotient of the field energy stored in theresonator and the power dissipation occurring in the resonator) is high(Q≈10⁷).

The positionally adjustable tuning plate according to the presentinvention can also, in principle, be a wall element (for example thecover wall) of the cavity resonator. A particularly preferred exemplaryembodiment of the invention is, however, characterized in that adielectric element is provided in the resonator body; and that thetuning plate is arranged inside the resonator body at a small distance dfrom a flat surface of the dielectric element. With a design of thiskind, much of the field energy is stored in the dielectric element, anda precise change in the resonance frequency of the cavity resonator canbe achieved by means of a change in the position of the tuning plate.

When a dielectric element is used, a further variant implementation thatis advantageous in terms of design consists in mounting the dielectricelement on a displaceable base whose height can be modified by means ofa second electromagnetic actuating element, in particular a secondpiezoelement. It is thereby possible, without great effort, to define adesired nominal or initial distance between the tuning plate and theflat surface of the dielectric element, which can then be finelyadjusted in suitable fashion by the adjustment device according to thepresent invention with downstream conversion ratio mechanism.

The invention will be explained below by way of example with referenceto an exemplary embodiment, with the aid of the drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectioned depiction of a cavity resonatoraccording to the present invention;

FIG. 2 shows a block diagram of a microwave oscillator that uses thecavity resonator shown in FIG. 1; and

FIG. 3 shows a diagram in which the change in oscillator frequency Δf isdepicted as a function of the change in position Δx₂ of the tuningplate.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a cavity resonator 1 of cylindrical design having aresonance frequency W_(R) in the GHz range. Cavity resonator 1 has abottom plate 2 in the shape of a circular disk, a cylindrical peripheralwall 3, and a cover wall 4. Resonator wall elements 2, 3, and 4 are madeof a metal having good electrical conductivity, for example Cu or anHTSL material, and define in their interior a cavity 5.

Bottom plate 2 has, distributed over its circumference, passthroughholes 6 through which pass threaded bolts 7 with which bottom plate 2 isfastened to a bottom flange 8 of peripheral wall 3. Arranged betweenbottom plate 2 and flange 8 is a spacer element 9 of predefinedthickness in the shape of an annular disk, and above it a displaceablebase 10 in the shape of a circular disk.

A multi-layer piezoelement 11 is located in the central region betweenbottom plate 2 and displaceable base 10. Multi-layer piezoelement 11 hasa maximum excursion of a few μm, which can be transferred todisplaceable base 10 and brings about a central bulging of the latter.

In the central region above multi-layer piezoelement 11, a dielectricpedestal element 12 that carries a dielectric cylinder 30 is arranged ondisplaceable base 10. Dielectric cylinder 30 is made of a dielectricmaterial having a high dielectric constant ∈ (for example, sapphire),and is arranged coaxially with peripheral wall 3 of cavity resonator 1.

A coupling-in antenna 13 a and a coupling-out antenna 13 b projectthrough the cylindrical peripheral wall 3 into cavity 5. Coupling-in andcoupling-out antennas 13 a, 13 b are each embodied as coaxial cableshaving coaxial loops configured at the ends.

Cover wall 4 of cavity resonator 1 is spaced away from a cover-sideflange 15 of peripheral wall 3 by means of a spacer element 14 ofpredefined thickness in the form of an annular disk, and is secured tocover-side flange 15, in a manner similar to bottom wall 2, by way ofthreaded bolts 17 passing through passthrough holes 16.

A comparatively coarse preadjustment of the resonance frequency ω_(R) ofcavity resonator 1 can be performed by using spacer elements 9, 14 withvariable thicknesses.

A plate spring 18 configured in the form of a thin metal disk is securedat the rim between annular disk-shaped spacer element 14 and cover wall4. In its central region, plate spring 18 delimits a cylindrical springreceiving space 19 present in cover wall 4. In the example depictedhere, spring receiving space 19 contains three cup springs 20, arrangedone above another, which are mounted around a central guide element 21and are braced at the bottom against plate spring 18.

Located above cover wall 4 is a micrometer screw 22 that comprises ascrew casing 23 joined immovably to cover wall 4, and a rotary member 24guided therein in a fine-pitch thread. Rotary member 24 impinges, withan actuating pin 24 a protruding at the bottom end, upon the upper endof a plunger 25, guided in a central bore of screw casing 23, whoselower end impinges upon a first multi-layer piezoelement 26 that acts onthe upper cup spring 20.

When rotary element 24 is displaced, plunger 25 is moved in the axialdirection with high adjustment accuracy (for example, 50 μm perrevolution). The movement travel is transferred to first multi-layerpiezoelement 26 and can be additionally modified, i.e. shortened orlengthened, by it. The linear excursion Δx₁ occurring at the output endof first multi-layer piezoelement 26 acts on the topmost cup spring 20and compresses it. Cup springs 20 press on plate spring 18 and deflectit in its central region over a deflection travel Δx₂. Because of theopposing force exerted by plate spring 18, the output-end deflectiontravel Δx₂ is smaller than the input-end linear excursion Δx₁. Thereduction in the deflection travel Δx₂ as compared to Δx₁, is determinedby the spring constant k₁ of the cup spring stack and the springconstant k₂ of plate spring 18.

If the spring constants are identical (k₁=k₂), the result is to shortenthe movement travel by a factor of 2.

A tuning disk 28 is mounted by way of a rod 27 on the side of platespring 18 facing away from spring receiving space 19. Tuning disk 28extends parallel to and at a short distance d from a flat surface 29 ofdielectric cylinder 30. A central deflection ΔX₂ of plate spring 18toward the bottom end causes tuning disk 28 also to be displaced by adistance Δx₂ so that a previously adjusted distance d between tuningdisk 28 and cylindrical body 30 is shortened to d−Δx₂.

FIG. 2 shows, in the form of a block diagram, the general constructionof a microwave oscillator that uses cavity resonator 1 depicted in FIG.1. An amplifier signal 41 of an amplifier 40 is conveyed to ahigh-frequency coupler 42. High-frequency coupler 42 on the one handcouples a useful signal 43 out of amplifier signal 41, and on the otherhand sends amplifier signal 41 on to cavity resonator 1. The coupling ofamplifier signal 41 into cavity resonator 1 is accomplished via inputantenna 13 a.

An output signal 44 is coupled out of cavity resonator 1 via outputantenna 13 b and conveyed to an electrically or mechanically actuablephase shifter 45 which is provided in order to adjust the phasecondition in feedback path 41, 42, 1, 44, 45. The phase-shifted feedbacksignal 46 generated by phase shifter 45 is fed into amplifier 40.

As already mentioned, the microwave oscillator can be continuously tunedonly if cavity resonator 1 achieves a requisite resonance frequencyadjustment accuracy Δω_(R) of approximately 100 kHz or less. It isunfavorable in this context that the tuning slope Δω_(R)/Δx₂ of a cavityresonator increases in proportion to its quality Q. With resonators 1 ofcomparatively low quality (Q≈10⁴), a typical tuning slope of 10 kHz/μmis observed. This means that the adjustment accuracy of the tuningmechanism, in terms of the achievable positional accuracy of tuningplate 28, needs to be only approximately 10 μm in order to achieve therequisite tuning accuracy Δω_(R) of 100 kHz for the resonance frequency.

The tuning slope for a quality Q≈10⁷, on the other hand, is already 10³kHz/μm. A quality Q≈10⁷ can be achieved, in the context of cavityresonator 1 according to the present invention, by cooling the latter toapproximately 77 K, since this allows the dielectric losses occurring indielectric cylinder 30 for so-called whispering gallery modes to begreatly reduced. In order to achieve continuous tunability of amicrowave oscillator with the cooled cavity resonator 1, the tuningmechanism of cavity resonator 1 must then have an adjustment accuracy of0.1 μm.

Conversion ratio mechanism 18, 20 depicted in FIG. 1 makes it possibleto achieve such adjustment accuracy when a micrometer screw 22 having anadjustment accuracy of 50 μm per revolution is used, and thus allowsimplementation of a completely tunable microwave oscillator having acavity resonator 1 with a quality Q≈10⁷.

The high adjustment accuracy of tuning mechanism 22, 20, 18 is due notonly to the reduction according to the present invention in movementtravel by way of conversion ratio mechanism 18, 20, but also to the factthat because conversion ratio mechanism 18, 20 is constructed fromspring elements placed one behind another, practically no backlashoccurs in it. This additionally makes possible excellent reproducibilityfor the adjustment position.

A further essential advantage of tuning mechanism 22, 20, 18 is itsmechanical stability and vibration resistance, especially at relativelylow excitation frequencies (<1 kHz). This is due not only theaforementioned robust and substantially zero-backlash design ofconversion ratio mechanism 18, 20, but also on the one hand to the highnatural mechanical frequencies of plate spring 18 and on the other handto the large forces that must be applied in order to deflect it (forexample, k₂=5000 N/m). An extremely low susceptibility to “microphoning”is thereby achieved, and even if cavity resonator 1 is cooled by meansof a commercial miniature cooler 1 [sic], no transfer of coolervibrations into the resonance frequency spectrum is observed.

Preferably the first and second multi-layer piezoelements 26, 11 canalso be used for electrical adjustment of the resonance frequency ω_(R).In this context, first multi-layer piezoelement 26 causes a movement oftuning plate 28 relative to the stationary dielectric cylinder 30, whileoperation of second multi-layer piezoelement 11 results in a movement ofdielectric cylinder 30 relative to the stationary tuning plate 28. Inparticular, first multilayer piezoelement 26 placed upstream fromconversion ratio mechanism 18, 20 makes possible very accurate fineelectrical adjustment of resonance frequency ω_(R) and is thusparticularly suitable as an actuating element for regulating theresonance frequency ω_(R) in a closed-loop mode.

FIG. 3 depicts a diagram that elucidates the tuning behavior of theoscillator shown in FIG. 2 under the following exemplary conditions:Cavity resonator 1 is cooled to a temperature of 77° K., and has adielectric cylinder 30 made of sapphire. A micrometer screw 22 having anexcursion of 50 μm per revolution is used, as well as three cup springs20 and a plate spring 18 that is 1 mm thick (k₂=5000 N/mm). Tuning plate28 is made of sapphire and has a thickness of 0.5 mm. Tuning isperformed at a frequency of 23 GHz.

The Y axis shown on the left side of FIG. 3 depicts the change inoscillator frequency Δf as a function of the linear excursion Δx₂ oftuning plate 28, plotted on the X axis. A change of 0.75 mm in thelinear excursion Δx₂ corresponds to a frequency change of 45 MHz.

Under the conditions specified, a minimum mechanical change in theposition of tuning plate 28 of Δx₂(min)<0.2 μm is achieved. FIG. 3 showsthat for small distances d<0.3 mm between tuning plate 28 and dielectriccylinder 30, this corresponds approximately to a minimum change inresonance frequency Δω_(R)(min)=4 kHz. This frequency change attainableby mechanical retuning of cavity resonator 1 is thus much smaller thanthe maximum frequency variation of approximately 100 kHz that can beeffected by phase shifter 45, i.e. the condition mentioned initially forcontinuous tunability of the microwave oscillator is easily met.

The quality Q of cavity resonator 1, plotted on the Y axis shown on theright side of FIG. 3, is largely constant over the entire tuning rangeof the microwave oscillator, and in the example here is Q>2·10⁶. Evenduring an adjustment operation, practically no degradation occurs in thequality of cavity resonator 1.

What is claimed is:
 1. A tunable cavity resonator, comprising: aresonator body (2, 3, 4) defining a cavity (5); a tuning plate (28)whose position with respect to the resonator body (2, 3, 4) isadjustable and which influences the resonance frequency (ω_(R)) of thecavity resonator; and an adjustment device (22, 26) for mechanicallychanging the position of the tuning plate (28), characterized by aconversion ratio mechanism (18, 20) which couples the adjustment device(22, 26) to the tuning plate (28) in terms of movement and whichconverts a linear excursion (Δx₁) generated by the adjustment device(22, 26), at a predefined ratio (U), into a reduced linear excursion(Δx₂) that acts on the tuning plate (28), the conversion ratio mechanism(18, 20) comprising a first spring element (20) whose end toward theadjustment device is deflectable with the linear excursion (Δx₁)generated by adjustment device (22, 26), and a second spring element(18) which impinges with an opposing force on the end of the firstspring element (20) remote from the adjustment device.
 2. The tunablecavity resonator as defined in claim 1, characterized in that the firstspring element is formed from at least one cup spring (20); and that thesecond spring element is implemented by a plate spring (18) that isimmobilized at the periphery and impinged upon centrally by the cupspring (20).
 3. The tunable cavity resonator as defined in claim 2,characterized in that the resonator body comprises a cylindricalperipheral wall (3), a cover wall (4), and a bottom wall (2); acylindrical spring receiving space (19) coaxial with the peripheral wallaxis and containing a cup spring (20) is configured in at least one ofthe cover wall (4) and the bottom wall (2); and that the plate spring(18) is immobilized in its radially external region between a flange(15) of the peripheral wall (3) and the cover wall or bottom wall (4;2).
 4. The tunable cavity resonator as defined in claim 1, characterizedin that the adjustment device (22, 26) comprises a manually actuablemechanical actuating element, and a first electromechanical actuatingelement, downstream from the mechanical actuating element.
 5. Thetunable cavity resonator as defined in claim 3, characterized in thatone or more spacer elements (9; 14) of predefined thickness are arrangedbetween the flange (15) of the peripheral wall (3) and at least one ofthe bottom wall and the cover wall (2; 4).
 6. The tunable cavityresonator as defined in claim 1, characterized in that the tuning plate(28) is made of a dielectric material.
 7. The tunable cavity resonatoras defined in claim 1, characterized in that a dielectric element (30)is provided in the resonator body (2, 3, 4); and that the tuning plate(28) is arranged inside the resonator body (2, 3, 4) at a small distance(d) from a flat surface (29) of the dielectric element (30).
 8. Thetunable cavity resonator as defined in claim 7, characterized in thatthe dielectric element (30) is mounted on a displaceable base (10) whoseheight can be modified by means of a second electromechanical actuatingelement.
 9. The tunable cavity resonator as defined in claim 1,characterized in that at least one of a first and a secondelectromechanical actuating element (11; 26) receives an electricalcontrol signal, output by an activation circuit, by means of which thecavity resonator (1) is operated in a closed-loop frequency controlmode.
 10. The tunable cavity resonator as defined in claim 1,characterized in that the cavity resonator (1) is thermally connected toan external cooling device.
 11. A tunable microwave oscillatorcomprising a cavity resonator as defined in claim 1, said tunablemicrowave oscillator further comprising an amplifier (40) which outputsan amplifier signal (41) that excites the cavity resonator (1), and aphase shifter (45) which receives an output signal (44) coupled out fromthe cavity resonator (1) and makes available a feedback signal (46),phase-shifted with respect to the output signal (44), which is deliveredto an input of the amplifier (40).
 12. The tunable microwave oscillatoras defined in claim 11, characterized in that the cavity resonator (1)has a quality Q>10⁶; and that the conversion ratio mechanism (18, 20) ofthe cavity resonator (1) is designed in such a way that the minimumchange in the resonance frequency (Δω_(R)(min)) achievable by a minimumpossible displacement of the adjustment device (22) is less than themaximum frequency excursion (Δω_(R)) of the resonance frequency (OR)achievable by a displacement of the phase shifter (45).
 13. The tunablecavity resonator as defined in claim 4, wherein said manually actuablemechanical actuating element is a rotary actuating element (22).
 14. Thetunable cavity resonator as defined in claim 4, wherein said firstelectromechanical actuating element is a first piezoelement (26). 15.The tunable cavity resonator as defined in claim 6, wherein saiddielectric material is sapphire.
 16. The tunable cavity resonator asdefined in claim 8, wherein said second electromechanical actuatingelement is a second piezoelement (11).
 17. The tunable cavity resonatoras defined in claim 10, wherein said external cooling device is amechanical miniature cooler.
 18. The tunable microwave oscillator asdefined in claim 12, wherein said cavity resonator (1) has a qualityQ>10⁷.